The present disclosure relates to delta-sigma modulators having an integrator.
Currently, delta-sigma modulators are widely used as analog-to-digital (A-D) convertors. The A-D converters using a delta-sigma modulator are characterized by higher accuracy and lower power consumption as compared to Nyquist A-D converters due to noise shaping and oversampling techniques. Among others, continuous time delta-sigma modulators are superior in terms of bandwidth and operation speed.
In typical continuous time delta-sigma modulators, an input signal passes through a plurality of cascaded analog integrators (i.e., continuous time filters) and is quantized by a quantizer. The output of the quantizer is converted to an analog current signal by a digital-to-analog (D-A) converter to provide negative feedback to the integrators (see, e.g., Steven R. Norsworthy, Richard Schereier and Gabor C. Temes, “Delta-Sigma Data Converters Theory, Design, and Simulation,” IEEE press, 1997, pp. 1-6, and H. Inose and Y. Yasuda, “A Unity Bit Coding Method by Negative Feedback,” Proceedings of the IEEE, Vol. 51, No. 11, November 1963, pp. 1524-1535).
In order to improve accuracy of the delta-sigma modulators, it is necessary to increase the order of the continuous time filters to remove quantization noise. The order of the continuous time filters can be increased by cascading the number of integrators corresponding to the filter order. However, this requires a large number of operational amplifiers, causing an increase in power consumption and chip area. Thus, it is desirable that an integrator implementing a high order transfer function by a single operational amplifier be used for the delta-sigma modulators. A small, low power consumption delta-sigma modulator that includes an integrator achieving high order integration characteristics by a single operational amplifier is implemented by providing a high order RC low pass filter and a high order RC high pass filter in an inverting input portion and a negative feedback portion of the operational amplifier, respectively, and providing negative feedback of the output of a quantizer to each intermediate node of each filter (see, e.g., PCT International Publication No. WO/2009/133653).
This improved delta-sigma modulator requires the high order RC low pass filter in the inverting input portion of the operational amplifier in order to obtain high order integration characteristics. This increases the circuit area of the integrator and also the overall circuit area of the delta-sigma modulator particularly because a capacitive element has a large area. Moreover, this improved delta-sigma modulator requires a large number of D-A converters that provide negative feedback of the output of the quantizer to the intermediate node of the RC low pass filter, and it is difficult to design a D-A converter that is connected to the intermediate node having an unstable potential.
Therefore, there is a need for smaller, lower power consumption delta-sigma modulators and integrators that are preferable for such delta-sigma modulators.